All petrol engines require spark plug to start which provides a spark to ignite the air-petrol mixture,but how does a mixture of diesel-air ignite resulting combustion in diesel engine without spark plug?

when the piston moves up to conpress the a/f mixture it is at a much higher PSI than a gas motor causes it to light itself on fire, thus glow plugs to help warm it up on a cold start

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A diesel engine runs at much greater compression ratio than a gas engine. A typical gas engine will run at 8.0 to 1 to maybe 12.5 to 1 ratio and uses a spark plug to ignite the air fuel mixture in the cylinder. A diesel engine depending on if it is turbocharged or not may have a compression ratio from say 16 to 1 to upwards of 22 to 1 ratio. The air is compressed first which raises the temperature, then atomized diesel fuel is sprayed into the chamber causing combustion. At the peak of the compression stroke the temperature of the air charge can exceed 1000 degrees farenheit causing the air fuel mixture to combust without the aid of a spark plug.

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2004 FX4 Level II Ranger.
32x11.50x15 BFG A/Ts
Bama Xcal 2 programmer
K&N FIPK
Flowmaster 40 series single in/dual out with Magnaflow 3 1/2" stainless slash cut rolled edge tips
Zoom Kevlar Clutch
5 Spd. Manual
Leather Seats
Big sound system.

All this above is what makes the horrible clatter sound on say pre-99 powerstrokes, and pre-duramax for sure (I work on the dinosaurs in the army).

All the pressure and force from the high compression ration and the 1,000 plus degree temps igniting the fuel literally explodes thereby "forcing" the piston back down to complete the cycle. Most modern diesels are equipped with a "wait to start" light that allows the glow plugs to warm up prior to starting the engine via glow plug controller and attempting to make the starting process easier. (I have never had an easy time starting our 6.2/6.5 fleet on a bitter cold morning)

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1994 sun faded (New Mexico/Texas truck) maroon, regular cab, 2.3 5 speed & 3.73's. Bought w/ 227k and climbing.